Block natural rubbers (TSR) which have been used traditionally include about 1.2% by mass at most of a fatty acid having 16 to 22 carbon atoms such as linolenic acid, stearic acid, oleic acid, palmitic acid and the like. If a total amount of these fatty acids and a fatty acid (normally stearic acid) that is separately compounded as a crosslinking accelerator exceeds about 1.0 part by mass based on 100 parts by mass of a rubber component, there is a problem that an adhesion between a Cu/Zn composite metal plating layer of a cord and a rubber composition is inhibited.
It is noted that a fatty acid having 16 to 22 carbon atoms generally forms a crosslinking accelerating composite with a sulfenamide vulcanization accelerator, zinc oxide and sulfur, and is considered to be necessary in a certain amount for crosslinking rubbers.
Methods of using cobalt stearate rather than directly using a fatty acid have been used. However, since a bonding force between stearic acid and cobalt in cobalt stearate is a weak ion bond, cobalt is supplied to the plating layer while stearic acid is released into the rubber component, and thus the increase of stearic acid derived from cobalt stearate causes the above problem.
While JP 2014-080475 A and JP 2013-122038 A disclose a rubber composition for coating a steel cord into which a vulcanization activator is compounded, a content of zinc benzoate is insufficient and there is still room for improvement.